$J=0$ fixed pole and $D$-term form factor in deeply virtual Compton scattering

TL;DR

This paper reviews the role of the $J=0$ fixed pole in deeply virtual Compton scattering, linking it to the $D$-term form factor and discussing the universality hypothesis and its potential violations.

Contribution

It derives a sum rule connecting the $J=0$ fixed pole to the $D$-term form factor and discusses the conditions under which the universality hypothesis holds or is violated.

Findings

The $J=0$ fixed pole is related to the $D$-term form factor via a sum rule.

Universality of the $J=0$ fixed pole is equivalent to the $D$-term being given by an inverse moment sum rule.

Adding extra $D$-terms to GPDs can violate the fixed pole universality hypothesis.

Abstract

S.~Brodsky, F.~J.~Llanes-Estrada, and A.~Szczepaniak emphasized the importance of the $J=0$ fixed pole manifestation in real and (deeply) virtual Compton scattering measurements and argued that the $J=0$ fixed pole is universal, {\it i.e.}, independent on the photon virtualities \cite{Brodsky:2008qu}. In this paper we review the $J=0$ fixed pole issue in deeply virtual Compton scattering. We employ the dispersive approach to derive the sum rule that connects the $J=0$ fixed pole contribution and the subtraction constant, called the $D$-term form factor for deeply virtual Compton scattering. We show that in the Bjorken limit the $J=0$ fixed pole universality hypothesis is equivalent to the conjecture that the $D$-term form factor is given by the inverse moment sum rule for the Compton form factor. This implies that the $D$-term is an inherent part of corresponding generalized parton distribution (GPD). Any supplementary $D$-term added to a GPD results in an additional $J=0$ fixed pole contribution and implies the violation of the universality hypothesis. We argue that there exists no theoretical proof for the $J=0$ fixed pole universality conjecture.